Twin-spindle machining centers with horizontal spindle position (e.g., Huller Hille, nb-h-twin) or vertical spindle position (e.g., Werner DUOMATIK) with automatic tool changers but fixed spindle position have belonged to the state of the art. These machines differ from single-spindle machines by a further increase in production output while flexibility is preserved at the same time. Two workpieces are usually machined in parallel on these machines, and the workpieces are mounted on a device corresponding to the fixed distance between the spindles. The field of use of these machines is inherently limited due to the fixed distance between the spindles, and the simultaneous machining of a plurality of positions on one workpiece is possible in exceptional cases only. Changing the distance between the spindles on these machines results in high costs.
Another drawback of machines with fixed distance between the spindles is the change in the distance between the spindles, which is always present, is caused by heating and can be controlled to a limited extent only even by prior-art compensation measures, e.g., cooling.
Since the prior-art twin spindles have a common drive, it is also impossible to work with tools with different diameters at the same time, because it is often impossible in this machine design to compensate the inherently different circumferential velocities via adapted spindle speeds.
A machine tool with two work spindles which can be moved separately from one another in the vertical direction, wherein each spindle is fastened to separately driven, vertically movable devices, has been known from U.S. Pat. No. 5,336,025. This design is highly complicated and expensive. This is also true of the design according to JP 62138515 U, in which two work spindles located one on top of another can be moved independently from one another in the vertical direction via separately, vertically arranged spindles.
Furthermore, the supplying with tools of these twin spindles with, e.g., two tool magazines with horizontal arrangement of the spindles or chain magazines with two to four tool grippers in one transfer arm with vertical arrangement of the twin spindles is associated with considerable costs.